Defines the pileup model expression to be used for fitting a 1D dataset.
sherpa> PILEUP [<dataset range> | ALLSETS] = <pileupModelExpr>
<dataset range> = # (or more generally #:#,#:#, etc.) such that #
specifies a dataset number and #:# represents an inclusive range of
datasets; one may specify multiple inclusive ranges by separating them
with commas. The default dataset is dataset 1.
In CIAO 3.0 Sherpa, there is one pileup model
available: JDPILEUP,
developed by John Davis at MIT. The pileup model does not work for
pileup in dispersed spectra. Rather, the model was designed for
imaging pileup, including pileup by the gratings in zeroth order.
This model is used for fitting energy spectra only.
Note that:
-
To apply the JDPILEUP model,
an instrument model with ARF must be defined, and the ARF
must have a uniform grid.
-
The parameters of the JDPILEUP model
(in particular, alpha)
themselves do not indicate how much pileup occurred during
the observation. To retrieve this information, use the
command SHOW PILEUP,
based on the PRINT_KERNEL function of the
fitting application ISIS. See the example
below for an explanation of the output of SHOW PILEUP.
-
By default, if the model instance of JDPILEUP
has not previously been established, Sherpa
will prompt for its initial parameter values.
This prompting can be turned off using the
PARAMPROMPT OFF command.
To reset the pileup model stack, issue the command:
sherpa> PILEUP [<dataset range> | ALLSETS] =
Define a pileup model for use:
sherpa> DATA data/source.pi
The inferred file type is PHA. If this is not what you want, please
specify the type explicitly in the data command.
WARNING: statistical errors specified in the PHA file.
These are currently IGNORED. To use them, type:
READ ERRORS "<filename>[cols CHANNEL,STAT_ERR]" fitsbin
sherpa> PARAMPROMPT OFF
Model parameter prompting is off
sherpa> INSTRUMENT = RSP[myinst]("data/source.rmf", "data/source.arf")
The inferred file type is ARF. If this is not what you want, please
specify the type explicitly in the data command.
sherpa> POW[p]
sherpa> SOURCE = p
sherpa> JDPILEUP[jdp]
sherpa> PILEUP = jdp
sherpa> FIT
In this example, data is input and a power-law
model component is established and
then defined as the source model for fitting. The command
JDPILEUP[jdp]
establishes the
JDPILEUP model component and assigns it the name
jdp.
The command
PILEUP = jdp then defines this pileup model for use in fitting.
After fitting, display the pileup fractions with SHOW PILEUP:
sherpa> SHOW PILEUP
1: 0.224696 0.541445
2: 0.263513 0.304025
3: 0.206024 0.113808
4: 0.120808 0.031952
5: 0.0566711 0.00717652
6: 0.0221538 0.00134322
7: 0.00742312 0.000215494
8: 0.00217637 3.02503e-05
9: 0.000567189 3.77462e-06
10: 0.000133035 4.23895e-07
*** pileup fraction: 0.458555
Each row represents the number of photons per CCD frame.
The maximum number of rows is 20; here, there were no more than
10 photons piled together per frame.
The left column represents the percentage of frames with
the stated number of photons: here,
22.47% of frames contained a single photon, 26.35% contained
two photons piled together, etc. The right
column indicates the percentage of observed counts with
the stated number of photons: here,
54.14% of recorded counts were comprised of one photon,
etc.. The total pileup fraction is the sum
of the right column, excluding the first row: here, 45.85%
of observed counts actually contain two or more photons piled
together.
- sherpa
-
autoest,
background,
create,
create_model,
createparamset,
fit,
freeze,
get_defined_models,
get_model_params,
get_models,
get_num_par,
get_par,
get_stackexpr,
getx,
gety,
guess,
instrument,
integrate,
is_paramset,
jointmode,
kernel,
lineid,
linkparam,
mdl,
modelexpr,
modelstack,
nestedmodel,
noise,
paramprompt,
paramset,
rename,
run_fit,
set_par,
set_paramset,
set_stackexpr,
source,
thaw,
truncate,
unlink
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